The use of backlit viewboxes for viewing transparencies, particularly for medical transparencies, is known in the art. It is also known in the art to use viewboxes which incidentally emit polarized light. Examples of viewboxes which emit polarized light are shown in U.S. patent application Ser. No. 08/348,958 filed Nov. 28, 1994 and in a PCT publication PCT/WO91/10152, titled "Self Masking Transparency Viewing Apparatus" and published on Jul. 11, 1991, the disclosures of which are incorporated herein by reference.
An important issue in transparency viewing is minimizing the amount of light from areas not covered by the transparency. Owing to certain psychophysical properties of the human visual system, it is more difficult for a viewer to discern low contrast details at lighting levels different from those to which the viewers eye is adapted. Since the eye adapts to all the light entering the eye and not just to light that is at the view point, reduction in extraneous light enables the eye to better adapt to lighting levels in the transparency. This adaptation enables the full range of sensitivity of the eye to be applied to the low contrast details in the transparency. It can also be appreciated why transparencies are usually viewed in relatively dark rooms.
A previous application, "Self Masking Transparency Viewing Apparatus," published as PCT/WO91/10152 on Jul. 11, 1991 and assigned to the same assignee, describes a system using a CCD camera and a controller to detect at least the edges of a film transparency placed on an LCA (Liquid Crystal Array) based viewbox. In operation, images are acquired through the camera and then analyzed by the system. The controller then masks the backlighting to those portions of the film or of the display surface that are of no interest to the viewer. A similar and more advanced device is described in unpublished U.S. patent application Ser. No. 08/348,958 filed Nov. 28, 1994.
However, such devices are relatively complex and usually require replacing existing viewboxes with new viewboxes.
Transparencies for X-ray imaging are typically manufactured by depositing light sensitive materials on a plastic sheet and then cutting the sheet down to the required transparency size. Usually, the plastic sheet is taken from a continuous roll of stretched plastic and the transparencies, which are usually rectangular, are cut so that their edges are parallel to the edges of the plastic sheet.